Repercussions from the chance that the federal government may not be able to pay all its bills starting next week already are being felt across the U.S. economy.

Over the past week the cost for the federal government to borrow on a short-term basis has tripled, Treasury Secretary Jacob Lew told the Senate Finance Committee in an unusual early-morning hearing Thursday that began before financial markets had opened.

Stock markets later surged on the news of a possible short-term debt-ceiling deal. The whopping 323.09-point gain in the Dow Jones industrial average Thursday served to remind how closely the markets are watching the debt debate, and how failure to reach a deal now or after a short reprieve could drive down prices.

Notwithstanding the Dow’s best day since Jan. 2, there’s plenty of evidence that fears of what might happen next week already are roiling financial markets. Investors are demanding a premium to buy Treasury bills that mature in coming weeks.

They’re demanding a higher return on a short-term Treasury bill than they ask for a Treasury bond that matures over two years. That’s highly unusual, and the rate the government had to pay investors Tuesday on short-term bills that mature Oct. 31 had reached its highest level since October 2008, the height of the brutal U.S. financial crisis.

The $2.6 trillion money market, a source of important overnight funds for financial companies, already has been affected by the threat of no debt deal. Fidelity Investments, the nation’s largest money-market fund, confirmed Wednesday that it’s been quietly selling off its Treasury bills that mature between mid-October and early November. JP Morgan Investment Management said late Thursday that it’s been doing the same.

The government hit the $16.67 trillion debt ceiling in May and has been moving money between accounts to ensure the bills are paid. Those measures run out around Oct. 17, and Lew told lawmakers they’re playing with fire if they don’t act before next week.

“I very much fear that a miscalculation . . . could lead to an unintended but very severe consequence,” Lew said, noting that he’s seen swings in daily incoming revenue and that the government could default unintentionally on some of its obligations before Oct. 17, or after it.

Sen. Patrick Toomey, R-Pa., asked Lew to publicly guarantee bondholders that the government won’t default on Treasury bonds. “Senator, all the options are bad,” Lew said.

If Congress doesn’t act before Oct. 17, he warned, “we’re going to be . . . in uncharted territory.”

Lew suggested that the closer the nation gets to the deadline the more risk of panic in markets that erases wealth and harms the economy. It was a warning echoed by Sen. Maria Cantwell, D-Wash., who noted the 20 percent drop in 2011 during the last protracted fight over the debt ceiling.

“By Friday or Monday you could see . . . as much as a 25 percent drop in the stock market,” she said, citing discussions she’d had with market participants. “We don’t have to go to default. Just talk of default is causing the uncertainty we’re trying to avoid.”

Economists think the August 2011 showdown slowed economic growth that had been gaining momentum and harmed consumer confidence. With the government shutdown now in its second week, important data that serve as a barometer of economic health aren’t being published and analysts are flying blind.

One statistic that’s still being published is the weekly report on first-time claims for unemployment benefits. On Thursday this data point showed a sharp spike of 66,000 more claims than the previous week – 374,000 claims for the week that ended on Oct. 5. Some of this jump is attributed to ongoing government computer problems in California, the nation’s most populous state. The weekly number doesn’t yet count furloughed government workers seeking benefits. It does, however, reflect employees who work for government contractors that aren’t getting paid during the shutdown.

There also are worries that a U.S. crisis could go global. At the start of International Monetary Fund meetings in Washington on Thursday, Managing Director Christine Lagarde warned of “spillover effects” should Democrats and Republicans push the United States into a voluntary default on some of its obligations.

Should the Oct. 17 deadline pass, there’d be about $30 billion left on hand for the Treasury Department to pay the more than 80 million transactions it conducts on a monthly basis. Lew could expect to see $10 billion to $15 billion in revenue flowing into government coffers every day, but it would be enough to cover only about 68 cents of every dollar owed to bondholders, Social Security recipients, military families and so on.

The Treasury could try to prioritize who gets paid, but Lew frowned on that prospect.

“This system was not designed to be turned off selectively,” he said.

Lew repeatedly warned that it’s not just a matter of paying interest to bondholders to prevent a default on U.S. government bonds. Hundreds of millions of dollars worth of bonds are maturing in coming weeks, leaving investors with the decision of whether they’re confident enough to reinvest. These bonds are also used as collateral in all sorts of financial transactions, and if financial markets decide they won’t trade them until the government is funded, it could cause the markets to seize up.